The most important when you have multiple gatewys with low capacity is being able to create agregation without having to tell every single carrier to add a new gateway with 8 ports or something like that.
To do this you need a gatekeeper to administrate your gateways so the origination carriers (all of them) see only your gatekeeper and your gatekeeper takes care of reatempting or avoid sending calls to non authenticated devices in case on of this small gateways dies.
1) You need to keep a good ASR in your routes, to do so you need a system smart enough to avoid making you loose calls being send to gateways that are not responding withing your network
2) Reatempts due to gateway capacity overflow so the call can be allocated to a secondary gateway authenticated on the same gatekeeper
3) Nothing of this will help you with call duration there are some other manual check up procedures you have to do to asure all your ports are working.
Make sure you activate your chanel hunt on your PSTNTG as ascending-round-robin this will allow you at least to attept one single time per port and do not loose too many calls in case one of your ports goes bad or the telephone line stops working, this is very common when you use the gateway together with GSM cellular services as Tellular.
Let me know if you need some information about good gatekeeper configurations and hardware you may like to use.
PD: When you are hidden in this forums IP you usually receive crap for response make sure you get what you are seeking for.