Summons is one of those odd nouns that ends in –s in its singular form; so the plural is summonses.
Odder still is the use of summons as a verb, meaning to command someone’s appearance in a court of law by way of a summons. Verbs don’t typically end in –s either.
Not everyone is happy with this state of affairs: Glanville Williams wrote in Learning the Law, 11th ed. (1982) that ‘the horrible expression “summonsed for an offence” (turning the noun “summons” into a verb) has now become accepted usage, but “summoned” remains not only allowable but preferable’.
A little digging in the dictionary (Oxford, of course) shows, however, that summons is older than Williams suggests – if not entirely reputable.
The usage citations in OED go back as far as 1780, with a quotation from Martin Madan’s Thelyphthora: ‘A woman had but to summons her seducer before the judges’. Madan (1726-1790) was a barrister (and a clergyman), but a controversial one; Thelyphthora scandalised its readers in advocating polygamy as a remedy for the ‘female ruin’ brought about by what he called whoredom, fornication and adultery. Perhaps not the best authority, then?
Next in the citations comes a line from Dickens’s Nicholas Nickleby (1828), where the sadistic schoolmaster Whackford Squeers (oh, those Dickensian names!) ejaculates, ‘Say another word and I’ll summons you for having a broken winder’. The dialectical winder may suggest that summons, too, is non-standard.
And the final example is from the novels of Marie Corelli, a forerunner of Barbara Cartland and EL James (so not exactly an exemplar of great literature): ‘You can summons me … if you feel so inclined’ (God’s Good Man (1904)).
All in all, not an impeccable pedigree for the verb summons, but it isn’t as newfangled a word as Glanville Williams suggests. I agree, though, that to summon is the better way to go.