What you have would depend on what was put in at time of installation.
I have GSM Gateways/modems installed at three sites (3 PABXs)
On my Hicom I had spare TMCOW ports so I used those.
On my ISDX-l I had spare analogue trunk ports so I used those.
On my Alcatel PCX 4400 I had spare analogue trunk ports so I used those.
It depends on what you have and what you want to spend. You also need to look at your call traffic to determine which solution is best for you.
Hicom - 9 GSM devices (9 sim cards) taking ~35-40,000 calls per month
ISDX-l - 13 GSM devices taking ~ 40,000 calls per month
4400 - 4 GSM devices taking ~ 10,000 calls per month
So you can see that having 26 devices we are taking about 80-90,000 calls a month off of the landline and to the GSM network.
The devices I used are from
http://www.psitek.com/ of the Fusion 100 variety from a local supplier. There are many types and brands of GSM Gateways/Modems this is just the one I used.
Buying another PRI Interface and PRI type GSM gateway wasn't cost effective for us. Plus we use two different carriers so I had to create two different trunk groups and routes.
So a call made from the PABX to a mobile gets treated like a mobile to mobile call. Giving you big cost savings.