The answer to launch and boost phase detection of ballistic missile detection is a pretty complicated one. In the US' case, it involves several overlapping radar and satellite detection systems that have been in place since the 1970s and even as far back as the late 60s for some(although those have already been replaced, same for the 70s and 80s deployed equipment).
US BMD detection uses the following systems for early warning and detection:
1. OTH radars - these are huge radars that use T/R antenna fields measuring in the acres/hectares. They employ atmospheric bounce and very high compute power to pull data from an otherwise highly cluttered radar picture caused by atmospheric interference. These are very exotic and are unlikely to be sold even to close allies. Australia developed their own OTH radar with some US assistance and they have one of the most recent OTH systems in the Western world.
2. Satellite Detection - another system in place since the Cold War are the satellite detection systems that have been continuously launched and replaced since the said era. These are what constitute the true "Launch Phase" detection of the BMEWS /PAVE PAWS/SSPARS early warning and detection system of the US... well this and their AEWACS aircraft which are constantly patrolling near Russian and other airspace for detection of ballistic missile threats. But safe to say the satellites are the most definitive detection system in play, with geosynchronous and geostationary orbits, they use various sensors(optical, IR, hyperspectral, radar, etc) to stare at launch sites and potential mobile launchers to "see" if the potential enemy is gearing up for launch.
3. Surface-based Radar Stations - while these are indeed capable of detecting ballistic missile launches, the job of radars like SSPARS is to see missiles in Late/Ascent/Post-Boost Phase should the other sensors miss them. Using huge land-based solid state phased array radars constantly being updated since the 60s, and housed in geographically advantageous sites around the world(Greenland, the UK, Alaska and the Aleutians, as well as several in the continental US). Notice I said surface-based rather than land-based? This is because the US Navy also employs forward deployed floating X-band radar bases as well as coverage from their AEGIS destroyers, C2 ships, as well as their aircraft-based detection systems. However the Navy detection systems come into play more in the Ascent/Post-Boost and Midcourse Phases of a ballistic missile launch. This is why BMD missiles employed by AEGIS systems often intercept at very high altitudes of the ASCENT phase and only work if early detection at launch or boost was achieved.
4. Airborne Early Warning Systems - again the US has multiple systems in play for this to happen, namely AEW&C aircraft like the E-3 Sentry and E-2 Hawleye, as well as HALE drones like GlobalHawk. These aircraft however need to be flying at the right altitude and right times to be able to detect a boost-phase launch. They still rely in early satellite detection of launch prep to be able to get up and the air and begin monitoring possible launch attempts.
So... if the Philippines really wanted BM detection, it would very quickly become either an expensive exercise where we buy/develop tracking satellites, OTH, and very powerful SSPARS radars to cover missile threats or one wherein we allow/ask Uncle Sam to establish a specific SSPARS/UEWS radar site in the country in exchange for the ability to detect early those said launches.