I am just sharing a post to use of Nerd-vittel, where he had configured a GSM GOIP Gateway to use as SIP outbound calling. Please note using VOIP gateway is not yet authorized in Bangladesh as far as i know. BTRC prior permission and information sharing may be required.
however you can use CLI modification , which is also not permitted unless you have your own short-code approve from BTRC. In Bangladesh for domestic call center operation, IPTSP is very god and affordable channel. But unfortunately its true that ordinary people does not like numbers 096XX, If you want an alternative you can go for GSM VoIP Gateway. All i am saying here asterisk is capable of using GSM SIM cards and able to send and receive calls.