Further expanding. GPRS Tunneling Protocol (GTP)[1] is what gets used to connected to the provider's data/voice network. This could be over any medium (wifi, GSM, UMTS, or LTE). It's likely this was the cert protecting GTP-C's ipsec tunnel[2] as without the ability to signal, pretty much everything on the network goes down.

[1] https://en.wikipedia.org/wiki/GPRS_Tunnelling_Protocol

[2] https://cyber-defense.sans.org/resources/papers/gsec/securin...