Sorry, but my DH would not go out for a meal with his ex, and neither would I.
We are both still friends with our ex's on Facebook. Due to having friends in common, we both see our ex's at parties and sometimes meals out, and will chat with our ex's. We had DH's ex to our evening wedding, and went to hers. They have so many friends in common, we can't avoid her. Not that we try. I actually think she is very nice. They dated for nine years (from 16 to 25), but didn't have any children together.
Neither of us telephone our ex's though. It's ten years since they split, and about eleven years since I split with my ex, and there is no awkwardness. But a meal with just the two of us, would be an absolute no.
Apparently it was awkward when DH first started dating me, as they'd split twice before but got back together. I think his ex thought it would be the same with their last split, and was heartbroken, that he met someone else. We met a month after the split, and immediately started dating. I didn't know he'd come out of a long-term relationship at the time. The ex met her now husband, and father of her children, about six months after the split. I think it was easier, with any awkwardness, once she started dating someone else. Although she never tried to get back with DH, or cause any problems with us.