Alepo makes excellent VoIP billing adn recently they started offering IP-2-IP softswitch solutions. With softswitch it will be slightly higher your budget, but the company is reliable, they offer money back guarantee, leasing options, and they treat customers honestly and professionally also after the sale is done.
The company is been in business for a long time, it is based in USA and the support is from India, but it is good, fast, and is staffed 24×7 including toll free telephone support in English. You will never speak with someone with fake name, and you can always call your US based rep, who is fully accountable to you.
Alepo may not be the cheapest choice, but it is a safe and reliable choice.