Grandstream HT503 Combo FXS/FXO ATA Adapter - Radicall
Grandstream HT503 ATA/IAD 1 FXS 1 FXO Port SIP Voip gateway Adapter with Lan and Wan|gateway external laptop battery charger|adapter rcagateway voip - AliExpress
Knowledgebase - What do the terms FXS and FXO mean? | Amernet
VoIP Analog Telephone Adapter (ATA) | 2 FXS analog ports